Processes for producing metal parts from ferrous powders using powder metallurgy (P/M) techniques are well known. Such techniques typically involute mixing of ferrous powders with alloying components such as graphite, copper or nickel in powder form, filling the die with the powder mixture, compacting and shaping of the compact by the application of pressure, and ejecting the compact from the die. The compact is then sintered wherein metallurgical bonds are developed by mass transfer under the influence of heat. The presence of an alloying element enhancers the strength and other mechanical properties in the sintered part compared to the ferrous powders alone. When necessary, secondary operations such as sizing, coining, repressing, impregnation, infiltration, machining, joining, etc. are performed on the P/M part.
It is common practice to use a lubricant for the compaction of ferrous powders. It is required mainly to reduce the friction between metal powders and die walls. By ensuring a good transfer of the compacting force during the compaction stage, it improves the uniformity of densification throughout the part. Besides, it also lowers the force required to remove the compact from the die, thus minimizing die wear and yielding parts with good surface finish.
The lubricant can be admixed with the ferrous powders or sprayed onto the die walls before the compaction. Die-wall lubrication is known to give rise to compacts with high green strength. Indeed, die-wall lubrication enables mechanical anchoring and metallurgical bonding between particles during compaction. However, die-wall lubrication increases the compaction cycle time, leads to less uniform densification and is not applicable to complex shapes. Therefore, in practice, the lubricant is most often admixed to the ferrous powders. The amount of lubricant is adjusted according to a specific application. Its content should be sufficient to minimize the friction forces at the die walls during the compaction and ejection of the parts. The amount of lubricant should, however, be kept as low as possible in the case of applications requiring high density level.
On the other hand, admixed lubricant most often reduces the strength of the green compact by forming a lubricant film between the metal particles which limits microwelding. When complex parts or parts with thin walls are to be produced, as well as when green parts have to be machined, parts with a high green strength are required. There is thus a need for a lubricant that would enable the manufacture of high green strength parts.
Most of parts produced by the P/M industry are compacted at room temperature without heating the powder and/or the tooling. This process is named "cold compaction". Parts which are "cold compacted" can reach a temperature of 50 to 70.degree. C. at ejection due to friction at die walls. On the other hand, powder and tooling can also be moderately heated to temperatures up to 180.degree. C. when parts with high density and/or high green strength are required. This process is known as warm compaction. Warm compaction takes advantage of the fact that a moderate increase of the compaction temperature lowers the yield strength of iron and steel particles and increases therefore their malleability, leading to an increase of density for a given applied pressure.
However, the temperature used in warm compaction may alter the properties of the admixed lubricant and therefore affect their lubrication behavior and flowability during the compaction and ejection stages. Effectively, most of lubricants that are suitable for cold compaction cannot be used in warm compaction due to their very poor lubricating and flowability properties. The poor lubricating properties result in high ejection forces. give parts with bad surface finish of ejected parts and increase die wear.
Conventional lubricants used in cold compaction include metallic stearates as zinc stearate or lithium stearate, or synthetic amide waxes as N,N'-ethylenebis(stearamide) or mixtures of metallic stearates and/or synthetic amide waxes. Polyethylene (PE) waxes, like CERACER 640, commercially available from Shamrock Technologies, have also been suggested as lubricants for cold compaction. Like synthetic amide waxes, polyethylene waxes have the advantage to decompose cleanly so that compacted parts are left free from residuals after the sintering operation. Shamrock Technologies report the use of their polyethylene wax lubricants to improve the green strength of metallic or ceramic bodies. However, Klemm et al., Adv. Powder Metall. & Particulate Mater., Vol. 2, 51-61 (1993), report in a study evaluating various P/M lubricants that the polyethylene wax tested lead to such a bad lubrication during the ejection of parts (high level of stick-slip), that they had to reject the idea to use this type of lubricant. In fact, the lubrication performance of PE waxes may vary drastically depending on the type of polyethylene waxes used. Indeed, Thomas et al. disclose in U.S. Pat. No. 6,140,278 that high density PE lubricants, having a weight-average molecular weight between 2,000 and 50,000, offer good lubrication of die walls and allow to obtain lower ejection forces than conventional lubricants, including low-density PE such as those supplied by Shamrock Technologies. Besides, unlike conventional lubricants and low-density PE lubricants, high density PE lubricants maintain their very good lubrication properties when moderately heated and give low ejection forces and good surface finishes. They also allow to obtain relatively high green strength up to 7000 psi and green densities up to 98% of the pore free density, which is known as a practical limit for warm pressing applications. High-density PE lubricants disclosed in the Thomas et al. patent, supra, can therefore be used for warm pressing.
Vidarsson discloses in PCT Application WO 99/11406 the use of linear chain polyethylene waxes having molecular weights between 500 and 10,000 and a polydispersity Mw/Mn preferably lower than 2.5, and more preferably lower than 1.5 for cold and warm compaction. No detailed description of the polymers covered by this application except for the molecular weight, the polydispersity and the melting point is given. In particular, no reference is made in this patent to the polarity or the oxidation state of the PE chains. No reference to any commercial PE waxes is also given in this application.
The presence or absence of polar-oxidized functional groups is known in the polymer industries as a key characteristic of PE waxes. Indeed, as described in U.S. Pat. No. 3,155,644, (col. 1, lines 42-63), non-polar PE waxes, "due to their inert nature, display poor receptivity for other materials" while oxidized PE waxes display improved adherence between polyethylene structures and substrate materials and improve bond strength between polyethylene surfaces and other base materials including metals.
Several US patents were issued in years 1964-1969 for processes of production of oxidized high-density polyethylene waxes.
Accordingly, and as it will be further described below, polyethylene waxes vary with regard to their lubrication performance and with regard to their possible use as lubricant for cold and warm compaction applications. It will be shown that polar-oxidized high-density PE waxes offer significantly better lubrication and compaction properties compared to non-polar high-density PE waxes.